1. Field of the Invention
The present invention relates to a hydraulic pressure control device of an automatic transmission for a vehicle such as an automobile, and more particularly, to an improvement with respect to a system for supplying and exhausting hydraulic pressure to and from friction engaging means such as clutches and brakes incorporated in the automatic transmission.
2. Description of the Prior Art
The automatic transmission for use in a vehicle such as an automobile has a speed stage change-over gear mechanism, and a plurality of hydraulically operated friction engaging means to change over a route of transmittance of torque in said speed stage change-over gear mechanism among various options so as selectively to set up each one of a plurality of speed stages. The hydraulic pressure control device of such an automatic transmission comprises a plurality of change-over valves adapted to operate in combination so as to change over supply and exhaust of hydraulic pressure to and from said plurality of friction engaging means according to selection among said speed stages to be set up. The supply and exhaust of the hydraulic pressure to and from some of said plurality of friction engaging means are directly controlled by a manual shift valve which changes over selection among shift ranges such as R, N, D, 2and L, without passing through said plurality of change over valves. However, most and other friction engaging means are each finally connected to a certain exclusive port of a certain one of said plurality of change over valves so as to be supplied with or exhausted of hydraulic pressure through such each exclusive port according to selection among the speed stages to be set up.
In some of such friction engaging means it is desired that the speed of increasing the hydraulic pressure in the supply process thereof is different from the speed of decreasing the hydraulic pressure in the exhaust process thereof. In order to meet with such requirements it is known to incorporate a unidirectional flow resistance in a passage connecting such a friction engaging means with said certain port, or to incorporate a certain first flow resistance in a passage leading from said certain port and supplying the hydraulic pressure to said certain friction engaging means, while incorporating a second flow resistance different from said first flow resistance in a passage for draining said certain friction engaging means through said certain port.
A new type of automatic transmission proposed by Japanese Patent Application 62-195471 assigned to the same assignee as the present application has a plurality of clutches incorporated in parallel with one another in a power input portion thereof, wherein a first one of such clutches is engaged when the 1st, the 2nd and the 3rd speed stages are set up and is disengaged when the 4th speed stage is set up, while a second one of such clutches is engaged when the 3rd and the 4th speed stages are set up. In such an automatic transmission, when it is shifted down from the 4th speed stage to the 3rd or the 2nd speed stage, said first clutch which is disengaged in the 4th speed stage needs to be engaged. In this automatic transmission, said first clutch need not be necessarily engaged in the 3rd speed stage because it is backed up by a one way clutch which is automatically engaged as the engine increases its rotational speed in the process of downshifting from the 4th speed stage to the 3rd speed stage. Therefore, in the downshifting from the 4th speed stage to the 3rd speed stage it is rather desirable that the engagement of said first clutch is delayed. However, when such an automatic transmission is shifted down from the 4th speed stage to the 2nd speed stage in which the engagement of said first clutch is essential, said first clutch needs to be engaged quickly. In other words, in said automatic transmission, when said first clutch is engaged in the downshifting from the 4th speed stage to the 3rd speed stage with a time lapse for the engine to increase the speed of rotation thereof, it is required that the hydraulic pressure is supplied thereto at a moderate speed so as to accomplish a shockless, smooth engagement thereof, whereas in the downshifting from the 4th speed stage to the 2nd speed stage it is required that a quick supply of hydraulic pressure to said first clutch is available. Therefore, it is required that a certain one friction engaging means can be supplied with the hydraulic pressure at two different speeds for the engagement thereof.
In order to meet with such requirements, in Japanese Patent Application 63-224328 assigned to the same assignee as the present application it has been proposed to incorporate a combination of a change-over valve and a throttling means in a passage connecting such a certain clutch finally with a certain port of a certain speed stage change-over valve, said change-over valve being combined with the throttle means and adapted to vary the flow resistance of the passage according to the change-over state thereof, so that, when said certain clutch is supplied with the hydraulic pressure in the downshifting from the 4th speed stage to the 3rd speed stage, the flow resistance of the passage is made to be relatively high, whereas, when said certain clutch is engaged for the automatic transmission being shifted down from the 4th speed stage to the 2nd speed stage, the flow resistance of the passage is made to be relatively low.
In the automatic transmission for vehicles the number of speed stages to be automatically set up is generally three or more, while the control of each friction engaging means therefor is basically alternative, i.e. engagement or disengagement, apart from the timing control in the process of engagement or disengagement thereof. Therefore, the hydraulic pressure control is an alternative control between supplying hydraulic pressure to a hydraulic pressure chamber of each friction engaging means and opening the hydraulic pressure chamber toward drain. The change-over valves generally employed in the hydraulic pressure control circuits in the automatic transmissions are spool valves which are of an alternatively controlling type to change over passages according to which of the two alternative shift positions a spool thereof is shifted to. Therefore, in the conventional hydraulic pressure control devices of the automatic transmissions two or more change-over valves are combined in a series connection so that selections among three or more speed stages are available based upon the principle of multiplication like 2.times.2=4 or 2.times.2.times.2=8.
Thus, in the conventional art of combination of two or more change-over valves it is conceived that those change-over valves provide an integral "speed stage selection control box" which allows for selections among three or more speed stages, said box having a housing which defines a border thereof relative to the external means, said housing having a plurality of ports for hydraulic connections with respective friction engaging means, so that each such friction engaging means is supplied with hydraulic pressure or opened to drain through one of such ports exclusive thereto. Therefore, if any certain timing control is required in the supply and exhaust of hydraulic pressure with respect to a certain one of such friction engaging means, such a timing control is a control subsequent to the speed stage change-over control, i.e. a control to be done by a means other than said "speed stage selection control box", and accordingly, it has been considered that any means for such additional control may be incorporated in the passage connecting such a certain friction engaging means with such a certain port provided in said housing of said "speed stage selection control box" to be exclusive to said certain friction engaging means.
The improvement by the above-mentioned Japanese Patent Application 63-224328 is also based upon such a technical concept with respect to the hydraulic pressure control device for the automatic transmission.